this is possible, but just a note
if you have a tv tuner that can pause, fast forward, rewind etc. TV, then you'll have to disable that functionality to be able to use your SNES... otherwise you'll have a 1-3 second delay with the video, I think its called PVR or PVM or something
google it, game console tv tuner or something of the sort
i'd recommend just getting one of those boxes that convert the S-Video cables to DVI for your monitor and then you can just switch the input from your monitor's little button panel when you want to play SNES etc.